In the related art, there is a known AD converter including two oscillation circuits that generate a pulse signal having an oscillation frequency corresponding to a level of a received analog signal. In such an AD converter, the number of pulse signals generated by each of the two oscillation circuits for a predetermined period is counted by a counter. After that, a difference between count values of the two oscillation circuits is output as a digital signal. In this manner, the difference of the level between the analog signals received in the two oscillation circuits is converted to the digital signal.
In such an AD converter of the related art, each of the oscillation circuits may have a different oscillation frequency value of the pulse signal generated for the same level analog signal because of manufacturing variations of the oscillation circuits. For instance, an oscillation circuit is designed to generate a pulse signal having the oscillation frequency 10 MHz with respect to an analog signal of a specific level, but sometimes pulse signal having the oscillation frequency 11 MHz may be actually generated with respect to the analog signal of the specific level. Also, there is a case in which a bias component contained in the received analog signal may differ from the design value. In the event of such manufacturing variations of the oscillation circuits or the difference generated in the bias component, there is possibility that each AD converter may have a different digital value to be output during a no signal period when no signal component is contained in the analog signal.
Moreover, a value of the oscillation frequency of the pulse signal generated with respect to the analog signal of the same level may be different between the two oscillation circuits included in the AD converter, and the difference of the oscillation frequency may cause offset in the digital signal to be output. For instance, in the case where one oscillation circuit generates a pulse signal of 10 MHz with respect to an analog signal and the other oscillation circuit generates a pulse signal of 10.1 MHz with respect to the same analog signal, there is a difference of 0.1 MHz oscillation frequency between the two oscillation circuits. This difference of 0.1 MHz may cause the offset in the digital signal.
Thus, in the AD converter of the related art, there is a problem in which the digital signals output with respect to the same analog signal may be varied because of the manufacturing variations of the oscillation circuits or the offset caused by the difference of the oscillation frequency between the two oscillation circuits.